Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Solder wettable contact – lead – or bond
Reexamination Certificate
2003-08-18
2004-11-16
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Solder wettable contact, lead, or bond
C257S780000, C257S781000, C257S761000, C257S762000, C257S763000, C257S764000, C257S766000, C257S738000, C257S772000
Reexamination Certificate
active
06819002
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of Taiwan application serial no. 91125098, filed Oct. 25, 2002.
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to an under-ball-metallurgy layer. More particularly, the present invention relates to an under-ball-metallurgy layer structure capable of increasing the bonding strength between the bonding pad on a chip and a solder bump.
2. Description of Related Art
Flip chip bonding technique is a method of joining a chip with a substrate or a printed circuit board (PCB). The chip has an active surface with a plurality of bonding pads arranged into an array. Each bonding pad on the chip has an under-ball-metallurgy layer (UBM) and a solder bump. Hence, the solder bumps may connect with a corresponding set of contact pads on the substrate or printed circuit board when the chip is flipped over. Note that the flip chip technique is able to produce a high-pin-count package with a smaller overall area and a shorter circuit length. Consequently, most semiconductor manufacturers have adopted the flip chip technique to fabricate chip packages, especially the high pin count packages.
FIG. 1
is a schematic cross-sectional view of a portion of a conventional flip chip. As shown in
FIG. 1
, the flip chip
100
includes a chip
110
, an under-ball-metallurgy layer
120
and a plurality of solder bumps
130
(only one is shown). The chip
110
has an active surface
112
with a passivation layer
114
and a plurality of bonding pads
116
thereon. The passivation layer
114
exposes the bonding pads
116
. Note that the active surface
112
of the chip
110
is the side of the chip
110
having most active devices. The under-ball-metallurgy layer
120
is formed between the bonding pad
116
and the solder bump
130
to serve as a junction interface.
The under-ball-metallurgy layer
120
further includes an adhesion layer
122
, a barrier layer
124
and a wettable layer
126
. The adhesion layer
122
is fabricated using a material such as aluminum or titanium for boosting the bonding strength between the bonding pad
116
and the barrier layer
124
. The barrier layer
124
is fabricated using a material such as nickel-vanadium alloy for preventing the diffusion of metallic atoms from a metallic layer above the barrier layer
124
to a metallic layer below the barrier layer
124
and vice versa. The wettable layer
126
is fabricated using a material such as copper for boosting the wetting capacity of the under-ball-metallurgy layer
120
towards solder bump
130
material. Note that lead-tin alloy is normally used to fabricate the solder bump
130
due to its greater overall bonding strength. However, because of possible pollution of the environment, lead-free solder material is often adopted. In general, both lead-containing and lead-free material contains tin as a principle ingredient.
If copper is a major constituent of the wettable layer
126
, tin within the solder bump
130
may react with copper inside the wettable layer
126
during a reflow process and lead to the formation of inter-metallic compound (IMC), in other words, Cu
6
Sn
5
. Gradually, an inter-metallic compound (IMC) layer (not shown) is formed between the wettable layer
126
and the solder bump
130
. In addition, if nickel-vanadium alloy is a major constituent of the barrier layer
124
, tin within the solder bump
130
may first react with copper inside the wettable layer
126
to form an inter-metallic compound (IMC) Cu
6
Sn
5
. Thereafter, tin within the solder bump
130
may continue to react with nickel within the barrier layer
124
to form another inter-metallic compound Ni
3
Sn
4
. Note that tin within the solder bump
130
and nickel within the barrier layer
124
react to produce inter-metallic compound Ni
3
Sn
4
, which is lumpy and discontinuous, after a long reflow process. In the presence of the lumpy inter-metallic compound, bonding strength of the solder bump
130
to the underlying under-ball-metallurgy layer
120
is weakened. Hence, the solder bump
130
may peel off from the chip leading to a deterioration of product reliability and yield.
SUMMARY OF INVENTION
Accordingly, one object of the present invention is to provide an under-ball-metallurgy layer between a bonding pad of a chip and a solder bump for reducing the growth rate of inter-metallic compound Ni
3
Sn
4
. Ultimately, bonding strength between solder bump and bonding pad can be maintained for a very long time and working life of associated chip package can be extended.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an under-ball-metallurgy layer structure between a bonding pad of a chip and a tin-based solder bump. The under-ball-metallurgy layer at least includes an adhesion layer over the bonding pad, a nickel-vanadium layer over the adhesion layer, a wettable layer over the nickel-vanadium layer and a barrier layer over the wettable layer. The barrier layer prevents the penetration of nickel atoms from the nickel-vanadium layer and reacts with tin within the solder bump to form inter-metallic compound. In addition, the barrier layer is fabricated using a material selected from a group consisting of nickel, iron and cobalt. Furthermore, the under-ball-metallurgy layer may include another wettable layer over the nickel-vanadium layer.
This invention also provides an alternative under-ball-metallurgy layer structure between a bonding pad of a chip and a tin-based solder bump. The under-ball-metallurgy layer at least includes an adhesion layer over the bonding pad, a wettable layer over the adhesion layer and a nickel-vanadium layer over the wettable layer. The nickel-vanadium layer may react with tin within the solder bump to form inter-metallic compound. Furthermore, the under-ball-metallurgy layer may include another wettable layer over the nickel-vanadium layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 6452270 (2002-09-01), Huang
Chang Chih-Huang
Chen William Tze-You
Cheng Po-Jen
Lee Chun-Chi
Tao Su
Advanced Semiconductor Engineering Inc.
Jiang Chyun IP Office
Loke Steven
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